LTE/IMS reference

 Basic LTE Attach Procedure

1. Each UE that is attached to the LTE network has at least one bearer available,that
is called default bearer. 

2. Its goal is to provide continuous IP connectivity towards the EPC (“ Always ON concept”)

3. From the Qos point of view, the default bearer is normally a quite basic bearer.

4. If a specific service requires more strength Qos attributes, then a dedicated bearer

    should be established. 

How many Default Bearers will be created- 

Normally 1 per PDN. That means if mobile equipment by default is connected to 5 PDNs,

then 5 default bearers will be created. One for each PDN. Normally only one.

Now, if mobile equipment is connected to all the 5 PDNs initially at the time of attach, E-NB

tell to MME that  mobile equip. want to connect to all the 5 PDNs, so MME talk to

corresponding PDN gateways which are connected to this PDNs and will have to create 5

EPS default bearer at the time of attach. Assume that these 5 PDN are connected to 5

different PDN gateways. Each PDN gateway will have one IP address to mobile phone.

From PDN gateway to SGW 5 tunnels will be created. But irrespectively of that how many

PDNs a mobile equip. is connected to, he must be connected to only one SGW. That

means single SGW mobile equip. is connected to 5 PDN gateway and 5 tunnels (S5/S8

bearers) will be created between SGW and PGW.

So ,After E-RAB establishment mobile equipment will have 5 IP address, 5 Radio bearers,

5 S1 bearers and 5 S5/S8 bearers and will be connected to 5 PDNs.

With these 5 existing RABs mobile equip. can actually send data to any application server

present in any 5 PDNs.

Now assume that suddenly one of the packet data network started with very high data rate 

call. That means default bearer is not enough, so dedicated EPS bearer needs to be 

created. While creating a dedicated bearer, we need to create one more tunnel b/w PGW 

to SGW (s5/s8 bearer) and SGW to E-NB (s1 bearer). E-NB to mobile equip. we need 

to create 6th Radio bearer which is having different Qos than the existing Radio bearers 

(so, mobile equip. knows by RRC Connection Reconfiguration message send by E-NB).

Now at the time of initial context setup request default bearer will be created but later if

dedicated bearer is getting created  then MME will send to E-NB that is E-RAB  setup and

after that E-NB will send same info. to mobile equipment  in RRC Connection

Reconfiguration  message.    

When many default bearers are created-

First scenario-  

If mobile equip. wants to connect 3 different packet data network like  IMS,INTERNET,

VPN. 3 default bearers need to be created even though Qos is same because there is

possibility that these three PDN can be connected to three different PDN gateways. 

Second Scenario- 

If mobile equip. wants to connect only one PDN like internet, but if 10 applications are

running, out of these 10 applications 8 are same Qos and 2 are same Qos. Then we need 2

default bearers even though only one PDN gateway is required.

So, Bearer concept depend on 2 things- one is Qos and other is PDN gateway. 

Dedicated can be one or more on high Qos basis.

EPS bearer Qos Attributes –

For every EPS bearer we must have below parameters -

ØTo be define per bearer

-Default / Dedicated bearer

- GBR/ N-GBR

- MBR

-UL/DL TFT(It is just like a port no. through which we can send the data corresponding

application of same EPS bearer or different EPS bearer.)

-QCI

- ARP          

(How long retain the bearer setup.

ØTo be define per User

- AMBR

QCI (Quality of service class identifier ) –

Every EPS bearer will have a QCI. One of the 9 QCI depending on what application we

use.

Attach procedure for the user-

The very first time when mobile equipment is switched ON, mobile equipment is in 
EMM_Deregistered State. 

1. Once UE is switched ON
2. PLMN search operation starts
3.UE NAS layer reads HPLMN information and frequency band gives it to RRC layer and
   asks RRC to find the PLMN.

4. RRC takes the help from physical layer to synchronize with different cells in the

    frequency band and read PLMN id.
5. RRC reports success to NAS if it finds the HPLMN.
6. NAS orders latching on to one of the cells in that PLMN.
7. RRC with the help of physical layer decides a cell to latch on

     based on signal strength comparison.

 8. UE must perform sync with selected cell.

      ........Cell Synchronization and Registration .............

ØUE should  read PSS (primary synchronization signal ) to find sector Id (0,1,2)

ØUE should  read SSS (Secondary synchronization signal ) to find Group Id (0- 167)

ØUE calculate PCI with 3*Group Id + Sector Id. synchronization.

 [ As soon as UE switches ON the first duty is synchronization.

 For synchronization we have two channels, one is PSS and the other one is SSS.

So, by reading primary synchronization signal mobile will have sector id (0,1,2). Similarly by

reading the secondary synchronization signal mobile find Group id (0-167). Once

the mobile find these two factors: sector id and group id, then mobile can find out PCI

(Physical cell id).   

PCI has got a formula that is-

PCI = 3* Group id + Sector id.

The PCI range is 0 to 503. Total 504 physical cell id . This cell is repeating to neighbor cell,

it may be repeating to far way cell. PCI is very much important in order to download

broadcasting data because all the broadcasting data and rest of the information will be

transmitted to the mobile equipment by using PCI only because if we wont do that the

broadcasting info. of this cell and broadcasting info. of next cell will be merged. Whenever

we will try to read the broadcasting channel, we will get the broadcasting channel data

which is a mixed information of this cell and the next cell. We don’t want that, so for all the

information's that are getting transmitted we must use PCI. Even in LTE we use same

frequency in neighbor cell also. So in order to separate the frequency from one single

antenna, we must use PCI. PSS and SSS are always present in middle PRBs. PSS and

SSS are always transmitted on the air by using the central 72 carriers/ central frequency

sub carriers always.

So, once we find out PCI, we need to do some kind of verification for the PCI.  Verification

can be performed by a special signal called a Reference signal (RS). This RS is used for

verification of PCI. Many reference signals are available in LTE frame architecture , So this

reference signal finds out what is the interference effect on LTE system ]    

Ø Verify  PCI  against the PCI found in reference signal.

ØNow UE’s physical layer reads the information PBCH.

In PBCH MIB information is present . Actually UE read to MIB to get the Bandwidth

(n6,n15, n25, n50, n100), PHICH and system frame number (SFN). SFN is very much

important to read SIB1. SIB1 will be present only in SFN mod 8=0 sub frame no.5. Actually

MIB is the RRC information. PHY layer won’t understand what is Band, what is SFN. PHY

layer just reads it but the info. is forwarded to MAC, MAC will forward to RLC, RLC will

forward to RRC. So finally RRC is the one who reads this information. So RRC get the MIB

info and he got to know what is the SFN. He will tell to PHY layer when to read the data

again. Next is read SIB1. SIB1 finds out the channel called PDSCH. But in order to read the

SIB1 info.we must be able to read a channel called as PDCCH. PDCCH gives which 

subcarrier is allocated for SI-RNTI. But in order to read PDCCH there are two other crucial

steps because UE need to understand how many symbols are reserved  for PDCCH as

PDCCH is not fixed. It is variable depending on the traffic (users). So, how many symbols

are reserved for PDCCH that information will be given by PCFICH. From the PCFICH UE

will get how many symbols are reserved for PDCCH  But problem is PCFICH is also not in

a constant position. So RRC layer find out where the PCFICH slots are and he will give

information to PHY layer and tell him that in next radio frame you will have to read

PCFICH. 

ØUE RRC  calculates which position and symbols are used for PCFICH 

because PCFICH  also dependent on PCI and Bandwidth. So UE RRC asks PHY layer to

read corresponding symbols in next radio frame,sub frame0 , slot 0. first symbol.

ØUE  gets information how  many symbols are reserved for  PDCCH channel.

ØUE reads PDCCH channel in sub frame  no.5 of SFN mod 8=0 and understand which

 resources are allocated for SI-RNTI.

ØUE should read SIB1 [ In PDCCH ]- SFN mod 8=0 sub frame #5.

ØUE decode the information received  on PRB indicated for SIB1. Once UE decodes 

PLMN ID, Tracking Area Code, Cell id cell barred, periodicity of other SIBS (SIB2

SIB12………rf3, rf16,  rf32, rf64 …..etc)

ØRRC gets to know the scheduling information for different SIBs. RRC provides this

information to PHY layer to read all the 12 SIBs.

ØBy following the scheduling information for SIBs, UE reads all the SIBs.

So, once UE reads all the 12 SIBs, first duty of UE is over. The first duty is to acquire all the

system information which is essential for all of his tasks.UE has successfully synchronized

and read full system information.But  at this stage UE  is still  EMM _ Deregistered  State .

From SIB1 UE acquire uplink channels like  which PRB  is allocated for PRACH, which

PRB is allocated for PUSCH, which PRB is allocated for PUCCH. UE knows by reading all

the system information.     

------Next job of UE is to perform Registration with MME----------  

 To do the registration, first UE should make a connection with E-NB.

ØNAS layer asks  UE’s RRC layer  to make a connection with E-NB.

ØRRC makes a message called RRC Connection Request.

This message  would come to RLC, RLC will be pumping it to MAC. Now

MAC will send it to E-NB with the help of PHY layer. So, out of  these uplink channel UE

should use PUSCH channel for sending the RRC Connection Request. In LTE  we wont

make RRC Connection for  resources. It is just connection  making. Resource  acquiring is

the job of MAC layer. That means RRC connection request message is waiting here and

UE will acquire the resource by performing Random Access Procedure (RACH). As soon

as the UE get resource from E-NB, then UE will send this request to E-NB.

ØSo, RRC message is waiting in MAC layer buffer until RACH is performed.

-------------RACH procedure to acquire uplink Resources---------------

There are two types of RACH

1. Contention Based ( User don’t have any ID , is use at initial time)
2. Non-Contention Based (User having ID, is use for handover )

Contention Based RACH procedure

Ø UE has a list of 64 preambles stored

ØMAC layer picks randomly one preamble

ØMAC gives physical layer this preamble data to be sent out in PRACH symbols

ØBased on sub-frame and slot of  PRACH used, UE’s physical layer calculate RA-RNTI.

RA-RNTI means Random Access- RNTI. Why RA-RNTI is used ?

Every user have own RA-RNTI. So by using different  RA-RNTI E-N decides which user is

used for which RA-RNTI for sending the resource on PDSCH channel.

RA-RNTI = 1+ t _id + 10 * f _id

t_id  = is the index of the first sub frame of the  specified  PRACH (0 < t_id <10 )

f_id = is the index of the specified PRACH with in that sub-frame. (0 < f_id <10 )

After sending this preamble information what UE will do? UE will keep monitoring PDCCH

channel and expecting that there is data getting in PDSCH channel in so on so PRB and in

so on so sub carrier.

ØE-NB also calculate the RA-RNTI and allocates the resources to UE and  need to send

this information to UE by using PDSCH channel as an Random Access Response (RAR) 

E-NB will send RAR by using RA-RNTI .

RAR = [ 1 Reserved bit + 11 bit Timing Advance Command + 20 bit uplink grant + 16 bit T

            CRNTI  ]

ØE-NB modulates this information on PDSCH and scheduling details are attached

to PDCCH by using RA-RNTI.

ØUE PHY layer reads this information (RAR) and forward to MAC.

ØNow UEs MAC layer sends the data (RRC connection req./ msg3) to PHY layer

by using allocated resource and send it to E-NB by using T-CRNTI on PUSCH. Now UE

start with contention resolution timer. with in the timer if E-NB have to confirmed to UE by

using UE contention resolution identity / msg4 (S-TMSI / Random value). That means UE

definitely will get RRC connection setup message from E-NB.

ØE-NB RRC receives this information (msg3) and he prepares RRC Connection

Setup. So this setup message comes from  RRC --à RLC --à  MAC. Now MAC needs to

send  it out.

ØMAC allocates PRB for UE to send this RRC message scheduling to be send to UE on

PDCCH by using T-CRNTI.

ØUEs PHY layer get this data and send it to MAC---à RLC--àRRC.

So, RRC  gets Setup message along with he gets one important message C-RNTI.

ØUE RRC prepares RRC Connection etup Complete  and send it to E-NB by using

allocated resource on PUSCH. This time UE should use C-RNTI.So, now onwards UE will

use C-RNTI .

RRC connection is established successfully 

ØNow , UE will send Attach request  to MME , NAS message actually. Actually  2 NAS

message  he will send-

1) EMM : Attach request

2) ESM : PDN connectivity request.

That means mobile is saying that please accept my attach request + give me PDN

Connectivity  request to my Default PDN. So E-NB initiates a   first NAS to  MME.

that is  ‘INITIAL  UE MESSAGE’. As soon as asked to MME, MME wont give it. MME have to verify with  HSS whether he subscribe for this PDN connectivity request or not.

ØNext is MME  will send to HSS that is  AUTHENTICATION   INFORMATION  REQUEST. 

Means please send me authentication related information for this user.

ØNow, HSS send the authentication related information in  AUTHENTICATION  

INFORMATION  ANSWER.

ØOnce the MME  gets this authentication information MME generating and sending the

NAS message to mobile and vice versa for the purpose validation (with XRES values) and

made a security (ciphering and integrity) in between UE and E-NB. 

ØNow MEE would send a message that is  UPDATE LOCATION REQUEST to HSS. So

MME updates to HSS that user is right now with me and asking for user subscription

related information, APN Configuration etc.

ØSo, HSS responded with UPDATE LOCATION ANSWER.

ØNow, MME decides and talk with SGW to create default bearer setup in EGTP Create

Session Request. And SGW will send same to PGW .

ØOnce this goes to PGW go and check with PCRF for Qos . PCRF give to PGW  Qos and

billing rule.

ØNow PGW send to SGW default bearer response in EGTP Create  Session Response.

And SGW forwards the same to MME.

ØNow, MME will send a message to E-NB that is INITIAL CONTEXT SETUP REQUEST.

ØSo, E-NB triggered to UE as RRC connection Reconfiguration for new radio bearer

establishment along with these 2 NAS messages.

ØSo, UE responded with rrc connection reconfiguration complete message and assuring

that he configured properties of radio bearer on all the layers. Here, E-NB understand that

radio part is created.

ØNow, E-NB replied with INITIAL CONTEXT SETUP RESPONSE  to MME.

ØAfter that UE will send 2 NAS messages that is EMM: Attach complete  and ESM: PDN

connectivity accept along with IP address assigned by PGW. So E-NB will forward the

same information to MME in UPLINK NAS TRANSPORT message.

ØNow MME will send to SGW that EGTP _ MODIFY BEARER REQUEST  and SGW

responded with EGTP _ MODIFY BEARER RESPONSE.

Now, the default bearer is created . There is radio bearer b/w UE and E-NB, tunnel

created in b/w E-NB to SGW and  SGW to PGW. Now all the 3 bearers are existing.

So default EPS bearer created for default PDN connectivity. 

ECM ( EPS Connection Management)-

When UE has established the both RRC Connection and S1 Connection, we say that ECM

connected that is between UE and MME.

It has two states-

1) ECM- Idle

2) ECM- Connected

Brief concept about States-

After moved to idle state----